I'll have to check the manual again, but I think the head is supposed to stay parallel to the ground. There is a "sweet spot", actually drawn on the head guard, that's supposed to provide the best cutting. This is a range of maybe 60+ degrees that's rotated somewhat to the right of straight ahead.
Now that I've reviewed the manual I can revisit this. I've noted that the manual is very good elsewhere, but here it provides little guidance about final adjustments. At the beginning of the short section titled Preparing the Trimmer/Mower for Trimming it says:
So trial and error is recommended, it seems.The characteristics of cutting height and quality is [sic] controlled by the position of the Trimmer Cords, location of the Locking Bolt, Wheel Spacer arrangement and height of Tractor PTO lift Arms. It takes time and practice to learn what the best combinations of settings for different conditions are.
Going by memory I somewhat mischaracterized the cutter's "sweet spot." The manual actually calls it the "Optimum Impact Zone," and it's where you're supposed to aim to hit obstacles such as fence posts. It's also more like 40 degrees than 60.